Dampers or torsional spike isolators have long been employed in conjunction with prior art transmissions for the purpose of absorbing or neutralizing torsional force aberrations or spikes emanating from the engine of the vehicle.
Torque converters are commonly employed in a vehicle to couple the engine to an automatic transmission. Torque converters employ a pump, commonly identified as an impeller, to direct hydraulic fluid through the torque converter. The impeller is rotated by the crankshaft of the engine, and a turbine is operatively attached to the input shaft of the vehicular transmission. A finned wheel, called a stator, is generally interposed between the impeller and the turbine to redirect the flow of hydraulic fluid within the torque converter.
The engine torque available at the turbine, which is transferred from the impeller to the turbine by means of the fluid coupling within the torque converter, can never quite be equal to the torque delivered from the impeller. This torque loss occurs because of inefficiencies in the fluid coupling, and the inefficiencies are evidenced by the rotational "slip" between the rotation of (or torque transfer from) the turbine with respect to the rotation of (or torque delivered to) the impeller. The fluid redirection accomplished by the operation of the stator in the torque converter almost overcomes this slip, but the ability of the turbine to slip with respect to the impeller serves to absorb the well known and recognized torsional spikes associated with the engine of the vehicle.
Because the stator does not completely overcome the slip, and to improve the efficiency of the transmission even further, a converter clutch--often designated as a "lock-up" clutch--is provided selectively to effect a direct drive between the impeller and the turbine portions of the torque converter. In those installations which incorporate a converter clutch to effect the desired direct drive across the torque converter during certain phases of the vehicular operation, the torque converter is precluded from absorbing torsional spikes while the converter clutch is operatively engaged. Thus, the utilization of a damper assembly within the torque converter itself allows the advantageous use of a converter clutch and also neutralizes the adverse affect of torsional spikes, even when a converter clutch is in use.
Standard transmissions, which operate without a torque converter, do not neutralize the adverse torsional spikes imparted by the engine without the inclusion of a component intended expressly for that purpose. Such installations generally employ a torsional damper within the drive train and locate the damper downstream from the engine.
Typical prior art damper assemblies have heretofore employed a plurality of compression spring members that absorb the transient torque spikes normally imparted to the crankshaft of the vehicle by the engine. One such assembly is disclosed in U.S. Pat. No. 5,009,301 issued to Spitler, Apr. 23, 1991, assigned to the assignee of the present invention.
To that end, it is desired to provide volute springs having a frictional hysteresis characteristic which makes them particularly suited for absorbing or neutralizing transient torsional vibrations or spikes. Frictional hysteresis provides a retarding effect to counteract the tendency of the spring to react over-promptly when the forces acting on the springs undergo virtually instantaneous changes.
While volute springs advantageously absorb spike loading, the prior known volute spring configurations suffer one primary drawback when used in applications requiring hundreds of thousands of compression cycles, such as occur when seeking to neutralize the torsional spikes imparted by an internal combustion engine. Specifically, fifty (50) to one hundred (100) percent of the coulomb (metal-to-metal) hysteresis characteristic in a standard volute spring is lost after being subjected to as few as 500,000 compression cycles. With the resulting loss of the coulomb hysteresis characteristic, the response time of the volute spring is too short to effect satisfactory damping of the loading spikes.
This reduction or loss of frictional hysteresis is due to wear "polishing" of the adjacent surfaces on the coils of the volute spring which rub together. That is, the continuous frictional contact between the adjacent surfaces on the coils of volute springs eventually eliminates the small surface grooves in the frictionally engaging surfaces.
In developing the present invention, it was determined that these small surface grooves allow lubricating oil to squeeze out between the layers of the mating surfaces of the volute spring, thereby maintaining the coulomb friction between the surfaces on the adjacent coils of the volute spring as they move with respect to each other. Without these surface grooves, however, a hydrodynamic oil film builds up between the adjacent coils. The loss of coulomb hysteresis occurs because the resulting hydrodynamic oil film does not permit the frictional contact necessary to effect frictional hysteresis.
The relatively low life span--e.g., 500,000 compression cycles--of the prior art volute springs makes it impractical for their usage in a mechanical device such as a damper assembly adapted for use in the drive train of a vehicle.